Vibrational Sum Frequency Generation Spectroscopy (VSFG) Studies of Organic Electronic Molecules

Abstract

This dissertation describes the application of vibrational sum frequency generation (VSFG) spectroscopy to the study of molecules utilized in organic solar cells (OSC). Two known molecules were chosen for this study, sexithiophene (6T) and fullerene (C60). In the first study VSFG is employed to probe C60 on dielectric surfaces. The SFG activity of this centrosymmetric molecule (theoretically SFG silent) is hypothesized to be from the surface perturbation, resulting an interruption in the symmetry and causing SFG activity. This suggestion is confirmed by experimental VSFG and calculations as well. This study also offers a unique method for estimating surface charge. In the second study the orientation of 6T molecules at different thicknesses is investigated using a combination of VSFG spectroscopy and atomic force microscopy (AFM). The results confirm previous X-ray studies that, 6T has two very different thickness-dependent orientations. At submonolayer thickness, 6T has an edge-on orientation which changes to an end-on orientation with increase in thickness. Additionally, using VSFG and the thin film interference model it is demonstrated that the orientation of 6T molecules are not the same on the two interfaces, 6T/air and substrate/6T. This study highlights the capability of SFG as a surface probe technique to analyze submonolayer thicknesses. The last study showcases electrical measurements on organic photovoltaic devices. The effect of different donor and acceptor materials and donor/acceptor modification through positioning a layer of a modifier molecule at the interface is examined.